1. Field of the Invention
The present invention is directed to a magnetic resonance apparatus, and in particular to a magnetic resonance apparatus of the type having an arrangement for preventing disturbances arising due to Lorentz forced produced by eddy currents from having a disturbing effect, such as noise generation of image quality degradation.
2. Description of the Prior Art
Magnetic resonance technology is a known technology to, among other things, acquire images of the inside of a body of an examination subject. For this purpose, in a magnetic resonance apparatus rapidly switched gradient fields that are generated by a gradient coil system are superimposed on a static basic magnetic field that is generated by a basic field magnet. The magnetic resonance apparatus also has a radio-frequency system that emits radio-frequency signals into the examination subject in order to produce magnetic resonance signals, and acquires the generated magnetic resonance signals on the basis of which magnetic resonance images are created.
A superconducting basic field magnet includes, for example, an essentially hollow-cylindrical helium vessel in which superconducting coils are arranged that are cooled by the fluid helium surrounding them. The helium vessel is enclosed by a hollow-cylindrical inner cryoshield that is in turn enclosed by a hollow-cylindrical outer cryoshield. The cryoshields are fashioned from a metal with good heat conductivity, for example aluminum. The cryoshields and/or the helium vessel are thereby kept at predeterminable temperatures by cryo-coolers, cold gas or liquid nitrogen. The outer cryoshield is ultimately enclosed by an essentially hollow-cylindrical vacuum vessel. The vessels are normally fashioned from non-magnetic stainless steel. The helium vessel is connected with the inner cryoshield, both cryoshields are interconnected, and the outer cryoshield is connected with the vacuum vessel. All connections are made in a minimally heat-conductive manner at a mutual separation of a few millimeters up to a few centimeters.
A hollow-cylindrical gradient coil system is attached in the cylindrical hollow of the vacuum vessel, for example by wedging into the hollow. To generate gradient fields, suitable currents are set in the gradient coil. The amplitudes of the required currents amount to more than 100 A. The current rise and fall rates amount to more than 100 kA/s. The basic magnetic field is on the order of IT and interacts with these temporally changing currents in the gradient coil and produces Lorentz forces, which lead to oscillations of the gradient coil system and therewith to unwanted acoustic noises and image quality interferences.
German OS 44 32 747 teaches an active measure for, in principle, reducing oscillations of the gradient coil system in a magnetic resonance apparatus.
For this, an apparatus, in particular employing electrostrictive elements, is arranged in or on the gradient coil system. With this apparatus, forces can be generated that counteract the oscillations of the gradient coil system, such that a deformation of the gradient coil system is substantially prevented. The electrostrictive elements are appropriately controlled for this purpose by an electrical voltage applied to them.
The gradient coil system normally is surrounded by conductive structures in which eddy currents are induced by the switched gradient fields. Examples of such conductive structures are the vacuum vessel and/or the cryoshields of the superconducting basic field magnet, a radio-frequency shield, for example made from a copper foil, and the antenna of the radio-frequency system. The fields as a consequence of the eddy currents are unwanted because without countermeasures they weaken the gradient fields and distort their time curve, which leads to impairment of the quality of magnetic resonance images.
The distortion of a gradient field as a result of the eddy current fields can be compensated to a certain degree by a suitable predistortion of a quantity controlling the gradient field. To compensate, the controlling quantity is filtered such that eddy current fields ensuing given non-predistorted operation of the gradient coil are cancelled by the predistortion. A filter network can be used for this filtering the parameters of which are determined by the time constants and coefficients that, for example, can be determined with a method described in German Patent 198 59 501.
By the use of an actively shielded gradient coil system, the eddy currents induced by the gradient coils fed with current (these eddy currents being on a predeterminable enveloping surface that, for example, runs through the inner cylinder jacket of the 80 K cryoshield of the superconducting basic field magnet) also can be reduced.
Furthermore, a radio-frequency shield provided with dividing slits is known from German OS 198 43 905 for a magnetic resonance apparatus. The radio-frequency shield, among other things, is slotted such that the eddy currents induced by the gradient fields in the radio-frequency shield are optimally suppressed.